1. Field of the Invention
The present invention relates to a surface acoustic wave element, and more particularly to a mounting structure and a mounting method for the surface acoustic wave element.
2. Description of the Related Art
As one example of conventional mounting structures and mounting methods of surface acoustic wave elements, Japanese Unexamined Patent Publication (JP-A) No. 150405/1992 discloses a flip-chip mounting structure and mounting method. FIGS. 1 and 2 show respectively the disclosed mounting structure and mounting method.
In the conventional mounting structure, a comb electrode 2 is formed by Al wiring on a functional surface 1a of a surface acoustic wave element 1, and Au bumps 25 are formed on connecting pads 4 of the surface acoustic wave element 1. The surface acoustic wave element 1 is disposed to face a mount surface 6a of a circuit board 6, and is electrically and mechanically connected to the circuit board 6. A silicone resin 26 having high viscosity is applied onto the surface acoustic wave element 1 by a dispenser and provides a coating to surround the surface acoustic wave element 1.
In the above conventional mounting structure of the surface acoustic wave element 1, since the silicone resin 26 covers an upper surface and periphery of the surface acoustic wave element 1, the mounting structure has an overall size greater than that of the surface acoustic wave element itself.
Also, the silicone resin 26 covers only the upper surface and periphery of the surface acoustic wave element 1, and an open space remains around the Au bumps 25 connecting the surface acoustic wave element 1 and board pads 7 on the circuit board 6. With such a structure, if the overall mounting structure is subjected to temperature changes repeatedly, stress is produced due to a difference in coefficient of thermal expansion between the surface acoustic wave element 1 and the circuit board 6 and directly impinges on joint portions of the Au bumps 25. Accordingly, the Au bumps 25 are susceptible to fatigue failure. The Au bumps 25 are also apt to break due to external impact, e.g., an impact caused in the event of being dropped.
Moreover, an epoxy resin and the like can be applied instead of the silicone resin 26 to cover the area around the surface acoustic wave element 1. Depending on viscosity of the resin applied, however, a difficulty is encountered in controlling the configuration of an oscillation space 9 defined between the surface acoustic wave element 1 and the circuit board 6. The reason is that the resin is forced to enter the oscillation space 9 by a capillary phenomenon and is deposited on an oscillation transmitting area of the functional surface la of the surface acoustic wave element 1. If such deposition of the resin occurs, desired characteristics of the surface acoustic wave element 1 are no longer obtained.
The conventional mounting method is carried out as shown in a flowchart in FIG. 2. First, the Au bumps 25 are formed on the functional surface 1a of the surface acoustic wave element 1 in the form of a wafer, i.e., on the connecting pads 4 which function as input/output terminals and a ground terminal (step S1) Then, the surface acoustic wave element 1 in the form of a wafer is subjected to dicing for separation into individual chips (step S2).
In a mounting step (step S3), the surface acoustic wave element 1 is mounted on the circuit board 6 so that the surface acoustic wave element 1 is positioned to face the mount surface 6a of the circuit board 6 with the functional surface la directed downward, and the metal bumps 25 on the surface acoustic wave element 1 are aligned with the board pads 7 on the circuit board 6.
Finally, the silicone resin 26 is applied to the backside of the surface acoustic wave element 1 (i.e., the side away from the functional surface 1a) (step S4). The mounting process is thereby completed.
The conventional mounting structure of the surface acoustic wave element 1 has the following problems.
First, the mounting structure has a size larger than that of the surface acoustic wave element. Accordingly, the surface acoustic wave element having the conventional mounting structure is not suitable for use in portable information equipment requiring electronics to be densely packed.
The above first problem is attributable to that surroundings of the surface acoustic wave element being covered by the silicone resin. The silicone resin functions to protect the surface acoustic wave element from the external environment and therefore cannot be omitted.
Secondly, in the conventional mounting structure, the silicone resin covers only the upper surface and periphery of the surface acoustic wave element, and an open space is left around the Au bumps connecting the surface acoustic wave element and the circuit board. Reliability, of connection is therefore low.
The above second problem is attributable to the situation that if the entire mounting structure is repeatedly subjected to temperature changes, stress is produced due to a difference in coefficient of thermal expansion between the surface acoustic wave element and the circuit board and is directly applied to the joint portions of the Au bumps. Accordingly, the Au bumps are susceptible to fatigue failure. The Au bumps are also apt to break due to external impact, e.g., an impact caused in the event of being dropped.
Thirdly, a difficulty is encountered in controlling the configuration of the oscillation space defined between the surface acoustic wave element and the circuit board depending on viscosity of the resin applied.
The above third problem is attributable to the situation that the resin is forced to enter the oscillation space by a capillary phenomenon and is deposited on the oscillation transmitting area of the functional surface of the surface acoustic wave element. If such deposition of the resin occurs, desired characteristics of the surface acoustic wave element are no longer obtained.
Additionally, similar mounting structures and mounting methods of surface acoustic wave elements are disclosed in Japanese Unexamined Patent Publications (JP-A) No. 307195/1996, No. 316778/1996, and No. 22763/1998.
An object of the present invention is therefore to provide a mounting structure of a surface acoustic wave element which is more compact.
Another object of the present invention is to provide a mounting structure of a surface acoustic wave element which is inexpensive and has high reliability.
In accordance with an aspect of the present invention, there is provided a mounting structure of a surface acoustic wave element in which a functional surface of the surface acoustic wave element is disposed to face a mount surface of a circuit board on which the surface acoustic wave element is to be mounted, and a gap between the functional surface and the mount surface is sealed off by a resin while an oscillation space is maintained between an oscillation transmitting area of the functional surface and the mount surface, wherein the resin is an anisotropic conductive resin, and the oscillation space is constituted by a space that is formed by the anisotropic conductive resin.
In accordance with another aspect of the present invention, there is provided a mounting method of a surface acoustic wave element in which a functional surface of the surface acoustic wave element is disposed to face a mount surface of a circuit board on which said surface acousitic wave element is to be mounted, and a gap between the functional surface and the mount surface is sealed off by a resin while an oscillation space is maintained between an oscillation transmitting area of the functional surface and the mount surface, the method comprising the steps of: placing an anisotropic conductive resin on the mount surface of the circuit board, the anisotropic conductive resin having a space serving as the oscillation space; positioning the functional surface of the surface acoustic wave element to face the mount surface of the circuit board in aligned relation; and connecting the surface acoustic wave element and the circuit board by heating the surface acoustic wave element to cure the anisotropic conductive resin while the surface acoustic wave element is pressed against the mount surface of the circuit board under pressure.